package org.apache.lucene.search;

/**
 * Copyright 2007 The Apache Software Foundation
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

import org.apache.lucene.index.IndexReader;

import java.io.IOException;

/**
 * Abstract base class providing a mechanism to restrict searches to a subset of
 * an index and also maintains and returns position information.
 * 
 * This is useful if you want to compare the positions from a SpanQuery with the
 * positions of items in a filter. For instance, if you had a SpanFilter that
 * marked all the occurrences of the word "foo" in documents, and then you
 * entered a new SpanQuery containing bar, you could not only filter by the word
 * foo, but you could then compare position information for post processing.
 */
public abstract class SpanFilter extends Filter {
	/**
	 * Returns a SpanFilterResult with true for documents which should be
	 * permitted in search results, and false for those that should not and
	 * Spans for where the true docs match.
	 * 
	 * @param reader
	 *            The {@link org.apache.lucene.index.IndexReader} to load
	 *            position and bitset information from
	 * @return A {@link SpanFilterResult}
	 * @throws java.io.IOException
	 *             if there was an issue accessing the necessary information
	 */
	public abstract SpanFilterResult bitSpans(IndexReader reader)
			throws IOException;
}
